1. Field of the Invention
The present invention relates to a satellite broadcast receiving system using a band switching filter and, more particularly, to a band switching filter circuit used as a bandpass filter for a second converter in, e.g., a satellite broadcast system.
2. Description of the Related Art
A conventional satellite broadcast receiving system includes a first converter for block down-conversion on a signal received by an outdoor parabolic antenna. The down converted signal is converted into a 1 to 1.5 GHz signal. A second converter is included for converting an output from the first converter into an IF frequency signal (e.g., 140 MHz).
The second converter includes a bandpass filter for band-limiting the obtained IF signal into a signal of 27 MHz in, e.g., Japan.
The bandwidth of the bandpass filter is fixed when a signal having a predetermined broadcast format is received, e.g., in a satellite broadcast service. However, the bandwidth must be set in correspondence with each signal source when a signal from a communication satellite is received. Therefore, in this case, the bandpass filter is needed to serve as a band switching filter.
A conventional band switching filter includes first and second bandpass filters connected in parallel between input and output terminals. The frequency characteristics of the first and second bandpass filters are different from each other. A pair of switching transistors receive a control signal and are alternately turned on/off to perform the band switching.
Each filter used in the band switching filter with the above arrangement must have high performance. When a high-performance filter is designed, the number of stages increases (e.g., in an LC filter) an LC filter. The number of comb electrodes in a surface acoustic wave filter increases also used therein is increased. Therefore, manufacturing cost of the conventional band switching filter having two high-performance filters is undesirably increased.
Since the switching transistors used in the switching filter correspond to the bandpass filters to perform band switching, the arrangement is undesirably complicated.